Answer: A
Step-by-step explanation: Domain is the input, or x values. You can see the points (1, 6), (2,3), (3,1), and (6,5). In ordered pairs, the x values are the numbers on the left:
(x, y)
So if we take all the numbers on the left, we get 1, 2, 3, and 6. Thus, our domain is {1,2,3,6}
